1. Field of the Invention
This invention relates generally to electrostatic samplers and methods used to collect aerosolized particulates. The particulates are collected on a dry collection surface or in a buffer solution or other liquid to facilitate subsequent processing and analysis. In particular, this invention provides a miniaturized electrostatic sampler designed for high efficiency and low energy (cost) collection of airborne particulates. The airflow through the sampler, electric fields, and collector geometry were obtained using physics-based computational optimization methods to maximize capture efficiency and selectivity for a target particle size while minimizing power consumption and device footprint.
2. Description of Related Art
The detection and analysis of aerosolized biological agents such as bacteria, bacterial, mold, and fungal spores, and viruses is desirable in a wide variety of settings including civilian environs such as hospitals, office buildings, and sports arenas, as well as military environments such as the battlefield, observation posts, and military housing. The ability to detect airborne particles such as bacteria and bacterial spores is critical to areas where accidental or deliberate release of harmful biological agents is suspected and can greatly help risk assessment and management, decontamination/neutralization and therapeutic efforts. Rapid detection of airborne pathogens can control the spread of bacterial infections in hospitals, schools, and animal facilities, for example.
In recent years, increasing concern has been expressed over the development of fast, accurate and robust countermeasures against the emergent threat of bioterrorism. A comprehensive review of biodetection technologies is provided in the following reference: NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES (2005) “Sensor Systems for Biological Agent Attacks: Protecting Buildings and Military Bases” Committee of Materials and Manufacturing Processes for Advanced Sensors, Board on Manufacturing and Engineering Design, Division on Engineering and Physical Sciences, The National Academies Press, Washington, D.C. Typically, the process of biodetection can be broadly sub-divided into the following steps: (1) sampling, where the airborne particles are captured into a suitable solid, liquid or gaseous matrix, (2) sample preparation, where the aforementioned matrix is processed to render the target entities in a format aligned with the downstream detector, and (3) sensing, where the target moieties in the sample are identified.
Reviews of bioaerosol sampling strategies are provided in the following references: National Institute of Justice (NIJ) Guide 101-00 (2001) “An Introduction to Biological Agent Detection Equipment for Emergency First Responders” US Department of Justice, Washington, D.C.; and HENNINGSON et al. (1994) “Evaluation of Microbiological Aerosol Samplers: A Review” Journal of Aerosol Science 25(8):1459-1492. Existing bioagent sampling technologies are largely based on (a) interception (such as filters), (b) inertial separation mechanisms (such as impingers, impactors, cyclones and centrifuges) or (c) electrostatic principles. Interception based aerosol samplers are a primarily intended for air purification and suffer from high costs of maintenance, difficulties in interfacing with analysis modalities and pre-determined cut-off size for sampling. Inertial separation mechanisms suffer from the disadvantages of high cost of operation, high power consumption, low collection efficiencies of viable microorganisms, and high cost of manufacturing/machining. Electrostatic precipitators, as opposed to samplers/collectors, are commonly used as air purifiers designed to filter air and not to capture airborne particulates on a substrate or matrix for analysis. Existing electrostatic samplers are too large for portable applications and use voltages that kill or damage many organisms, thus preventing or complicating organism detection and identification.
Recently, the use of electrostatic samplers for collection of airborne microorganisms was demonstrated by the following references: MAINELIS et al. (2002) “Design and Collection Efficiency of a New Electrostatic Precipitator for Bioaerosol Collection” Aerosol Science and Technology 36:1073-1085; and MAINELIS et al. (2002) “Collection of Airborne Microorganisms by a New Electrostatic Precipitator, Journal of Aerosol Science 33:1417-1432, which are incorporated by reference in their entirety. A simple design comprising a parallel plate electrode configuration was used for developing the proof-of-concept in these studies. Physical collection efficiencies of >90% and biological collection efficiencies of >70% were demonstrated for air flow rates up to 8 L/min. Electrostatic samplers use an externally applied voltage to charge particulates in the air and deposit them on a collection surface. The collection surface can be an electrode with a dry surface or an electrode covered with a stationary or moving liquid. Electrostatic collectors (samplers) that deposit particles in a liquid medium can be used to concentrate samples from the air and deliver them to fluid-based biological assay modules such as microfluidic chips for analysis. This format is particularly useful for detecting or identifying biological agents such as bacteria, viruses, and bacterial, mold, and fungal spores, for example.
U.S. Patent Publication 2004/0083790 (CARLSON et al.) describes a portable liquid collection electrostatic precipitator. The device comprises: a hollow, vertical, tubular collection electrode; a ground plate adjacent to the collection electrode; a reservoir for a liquid, a pump for pumping the liquid, and an ionization section to ionize analytes in the air. Particles in the air are ionized, attracted to the collection electrode, and precipitated in the liquid. The device described by CARLSON et al. uses a high voltage collection electrode of 6,000-8,000 volts to attract charged particles, an airflow rate of 300 L/min, and can be powered by a 12-volt automobile battery. High voltages such as those applied to the Carlson et al. collection electrode can kill many organisms and thereby prevent or make more difficult their detection and/or identification. In addition, the high voltage applied at the collection electrode, which is normally bathed in aqueous liquid, poses a significant safety hazard. The CARLSON et al. sampler does not disclose the collection of small diameter particles with high efficiencies or designs capable of miniaturization while maintaining high collection efficiencies.
There remains an unmet need in the art for a miniaturized, portable electrostatic air sampler that can collect particles, including viable airborne viruses and bacterial spores, with high efficiency.
The present inventors have applied physics-based computational fluid dynamics (CFD) analysis to design novel, miniaturized electrostatic samplers that occupy less space, consume less power, capture particles with higher efficiency, and have greater operational flexibility than existing electrostatic samplers/collectors. Several innovative concepts for high throughput sampling were identified and evaluated using coupled airflow, particle transport and electric field simulations. The optimized samplers have predicted collection efficiencies of >90% at <5,000V and 60 L/min, even for 1 μm particles. Clustering of collectors in an electrostatic sampler array can easily achieve airflow rates of 300-1000 L/min and higher. A high voltage outer electrode allows the use of a grounded collection electrode to maintain the viability of collected cells and spores. The outer and/or collection electrode may be segmented to form programmable electrode array(s) to enhance efficiency and to reduce the area onto which particles are deposited. Testing of a prototype electrostatic sampler design has verified the performance predicted by CFD simulations.